1. Field of Invention
This invention relates generally to the field of man-made earth satellites and more specifically to a novel method and associated apparatus for capturing, servicing and de-orbiting such satellites.
2. Background of the Invention
The last half-century has been a watershed for the development and implementation of earth-orbit satellites for various purposes. Different types of earth satellites include those designed for communications, earth remote sensing, weather, global positioning, and scientific research. A typical example of a communications satellite might be EchoStar 3, which is used to send television signals to homes throughout North America. Communication satellites act as relay stations in space. They are used to “bounce” digital messages, such as telephone calls, television pictures and internet connections, from one part of the world to another. EchoStar 3 and many other communications satellites are in geosynchronous orbit. There are more than 100 such communication satellites currently orbiting earth.
Earth remote sensing satellites, such as the LandSat series (LandSat 1 through LandSat 7), study the earth's surface. From 300 miles above the earth's surface, and more, these satellites use powerful cameras to scan the planet. Their instruments study earth's plant cover, chemical composition, surface temperature, ice cap thicknesses, and many other earth systems and features. Such data is useful in vital industries such as farming, fishing, mining, and forestry. Moreover, remote sensing satellites are used to study changes in global environments caused by man. Examples of this include areas that are turning into desert (desertification), and the destruction of rain forests (deforestation).
TIROS (Television Infrared Observational Satellite), operated by NOAA (National Oceanic and Atmospheric Administration), is a representative weather satellite. TIROS is one of several weather satellites making up a system operated by NOAA, which produces data used to forecast weather, track storms, and otherwise engage in meteorological research. There are two TIROS satellites circling earth over the poles while satellites from another part of the system, the Geostationary Operational Environmental Satellites (GOES), operate in geosynchronous orbit. Using this system of satellites, meteorologists study the weather and climate patterns around the world, such as temperature, moisture, and solar radiation in the atmosphere. Also, many weather satellites are equipped with sensors that aid search and rescue operations.
The Global Positioning System (GPS) satellites are in geosynchronous orbit and are able to identify latitude, longitude and altitude with great precision. Originally developed by the military, GPS satellites are now used by a wide variety of people everywhere to find their exact position. Airplanes, boats, cars and virtually any transportation apparatus are equipped with GPS receivers. Even hand-held GPS devices have become a norm with pedestrians and other travelers.
While there are many examples of satellites engaged in scientific research, Hubble Space Telescope (HST) is perhaps the best known. Since 1990, when HST was launched, the world has had access to remarkable visual images that have significantly advanced our understanding of the cosmos. HST's optics, science instruments and spacecraft systems work together to capture light from far reaches of the universe, convert it into digital data, and transmit it back to earth. Because application of the current invention to HST represents an exemplary mode contemplated of carrying out the invention at the time of filing for a United States patent, an overview of HST's systems is appropriate. However, one skilled in the art will recognize that the method of this invention can be applied and adapted to virtually any man made earth satellite.
HST optics are collectively called the Optical Telescope Assembly (OTA), which includes two mirrors, support trusses and the apertures of the accompanying instruments. OTA's configuration is that of a well-known, straightforward design known as Ritchie-Chretien Cassegrain, in which the two specialized mirrors form focused images over the largest possible field of view. Referring now to FIGS. 1 through 5, HST and OTA are graphically illustrated. While FIG. 1 shows a three dimensional cutaway view of HST, FIGS. 2 and 3 specifically illustrate OTA and the principles of its operation. As best seen in FIG. 3, light enters through the main aperture and travels down a tube fitted with baffles that keep out stray light. The light is collected by the concave primary mirror and reflected toward the smaller, convex secondary mirror. The secondary mirror bounces the light back toward the primary mirror and through a smaller aperture in its center. The light is then focused on a small area called the focal plane, where it is detected by the various science instruments.
OTA's mirrors are necessarily very smooth and have precisely shaped reflecting surfaces. They were ground so that their surfaces do not deviate from a mathematically perfect curve by more than 1/800,000 of an inch. According to this precision, if the primary mirror were scaled up to the diameter of the earth, the biggest bump would be only six inches tall. Shortly after HST was deployed, it was discovered that the curve to which primary mirror was ground was incorrect, causing a spherical aberration. Fortunately, corrective optics, much like a contact lens, were able to solve this problem.
The mirrors are made of ultra-low expansion glass and kept at a nearly constant room temperature (about 70 degrees Fahrenheit) to avoid warping. The reflecting surfaces are coated with a 3/1,000,000-inch layer of pure aluminum and protected by a 1/1,000,000-inch layer of magnesium fluoride, which makes the mirrors more reflective of ultraviolet light.
HST contains five science instruments, namely the Advanced Camera for Surveys (ACS), the Wide Field and Planetary Camera (WFPC2), the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), the Space Telescope Imaging Spectrograph (STIS), and the Fine Guidance Sensors (FGS). These instruments work either together or individually to observe the universe in a unique way.
Installed in the latest space shuttle servicing mission in March of 2002, the ACS represents the third generation of science instruments flown aboard HST. It occupies the space vacated by the Faint Object Camera, HST's “zoom lens” for nearly twelve years. Among other tasks, ACS is used to map distribution of dark matter, detect the most distant objects in the universe, search for massive planets in other solar systems, observe weather on other planets in our solar system, and study the nature and distribution of galaxies. With its wider field of view, sharper image quality, and enhanced sensitivity, ACS expands HST's capabilities significantly; its cutting edge technology makes HST ten times more effective and prolongs its useful life. Designed to study some of the earliest activity in the universe, ACS detects electromagnetic waves in wavelengths ranging from far ultraviolet to infrared.
On the inside, ACS is actually a team of three different cameras, specifically the wide field camera, the high-resolution camera, and the solar blind camera. The wide field camera conducts broad surveys of the universe, which reveal clues about how our universe evolved. In contrast, the high-resolution camera takes detailed pictures of the inner regions of galaxies. The solar blind camera, which blocks visible light to enhance ultraviolet sensitivity, focuses on hot stars radiating in ultraviolet wavelengths.
HST's “workhorse” instrument, WFPC2, is behind most of the famous images it produces. This main camera includes 48 filters mounted on four filter wheels, which allow scientists to study precise wavelengths of light and to sense a range of wavelengths from ultraviolet to near-infrared light. Four postage stamp-sized pieces of circuitry called Charge-Coupled Devices (CCDs) collect and record information from stars and galaxies to make photographs. These detectors are very sensitive to the extremely faint light of distant galaxies. In fact, CCDs can see objects that are one billion times fainter than the human eye can see. Less sensitive CCDs are now found in some videocassette recorders and virtually all new digital cameras. Each of the four CCDs on HST contains 640,000 pixels. The light collected by each pixel is translated into a number. These numbers are then transmitted to ground-based computers, which convert them into an image.
NICMOS is HST's “heat sensor” and can see objects in deepest space—objects whose light takes billions of years to reach earth. The instrument's three cameras, each with different fields of view, are designed to detect near-infrared wavelengths, which are slightly longer than the wavelengths of visible light. Much information about the birth of stars, solar systems, and galaxies are revealed in infrared light, which can penetrate the interstellar dust and gas that often block visible light. In addition, light from the most distant objects in the universe “shifts” into the infrared wavelengths, and so by studying objects and phenomena in this spectral region, astronomers can probe the past, learning how galaxies, stars and planetary systems form.
Just as a camera for recording visible light must be dark inside to avoid exposure to unwanted light, so must a camera for recording infrared light be cold inside to avoid unwanted exposure to unwanted light in the form of heat. To ensure that NICMOS is recording infrared light from space rather than heat created by its own electronics, its sensitive infrared sensors must operate at very cold temperatures—below 77 degrees Kelvin (−321 degrees Fahrenheit). The instrument's detectors were initially cooled inside a cryogenic dewar (a thermally insulated container much like a thermos bottle), which contained a 230 pound block of nitrogen ice. While successful for about two years, the nitrogen ice cube melted prematurely. NICMOS was re-chilled during the last HST servicing mission of March 2002 with a “cryocooler,” an apparatus that operates much like a household refrigerator.
STIS in essence acts like a prism to separate detected light into its component colors. This spectrograph instrument thus provides a “fingerprint” of the object being observed, which reveals information about its temperature, chemical composition, density and motion. Spectrographic observations also show changes in celestial objects as the universe evolves. STIS spans ultraviolet, visible and near-infrared wavelengths. Among other tasks, STIS is used to search for black holes. The light emitted by stars and gas orbiting the center of a galaxy appears redder when moving away from earth (redshift) and bluer when coming toward earth (blueshift). Thus, STIS looks for redshifted material on one side of the suspected black hole and blueshifted material on the other, indicating that this material is orbiting at a very high rate of speed, as would be expected when a black hole is present. STIS can sample 500 points along a celestial object simultaneously, meaning that many regions in planet's atmosphere or many stars within a galaxy can be recorded in one exposure. STIS was installed on HST during the 1997 shuttle servicing mission.
HST's Fine Guidance Sensors, its targeting cameras, provide feedback used to maneuver the telescope and perform celestial measurements. While two of the sensors point the telescope at a desired astronomical target, and then hold that target in an instrument's field of view, the third sensor is able to perform scientific observations. The FGS aim HST by locking onto “guide stars” and continuously measuring the position of the telescope relative to the object being viewed. Adjustments based on these constant, minute measurements keep HST pointed in the desired direction with an accuracy of 0.01 arcsec. The FGS detect when HST drifts even the smallest amount and return it to its target. This gives HST the ability to remain pointed at that target with no more than 0.007 arcsec of deviation over long periods of time. This level of stability and precision is the equivalent of being able to hold a laser beam focused on a dime 200 miles away for 24 hours.
Additionally, FGS provide precise astrometrical measurements of stars and celestial objects, which are advancing the knowledge of stars' distances, masses and motions. FGS provide star positions that are about ten times more accurate than those observed from ground-based telescopes. When used as science instruments, the sensors allow HST to search for a “wobble” in the motion of nearby stars, which may indicate that they have planets orbiting around them; determine if certain stars are actually double stars; measure the angular diameter of stars and other celestial objects; refine the positions and the absolute magnitude (brightness) scale for stars; and help determine the true distance scale for the universe.
All telescopes have optical systems, and some even have specialized instruments, but HST is almost unique in that it operates in space; the telescope is actually “flown” as a spacecraft. Therefore, several space craft systems are required to keep HST functioning smoothly. The essential systems are communications antennae, solar arrays for power, computers and automation, and housing.
HST performs only in response to detailed instructions from a ground-based control center, and thus communications antennae are necessary to transmit and receive such instructions between the telescope and the Flight Operations Team at the Space Telescope Science Institute. The four antennae on HST transmit and receive data via one of the constellation of Tracking and Data Relay Satellites (TDRS) operated by NASA. In order for this system to be operational, at least one TDRS satellite must be visible within HST's line of sight. Direct interaction can occur between HST and the control center only when this line of sight exists. When none of the TDRS satellites are visible from HST, a recorder stores the accumulated data until visibility is resumed. A flow diagram of the communications process is provided as FIG. 4.
Flanking HST's tube are two thin, blue solar panel arrays. Each wing-like array has a solar cell “blanket” that converts the sun's energy directly into electricity to power HST's various systems. Some of the energy generated by the arrays is stored in onboard batteries so that HST can operate while traveling through earth's shadow (about 36 minutes out of each 97 minute orbit). Fully charged, each battery contains enough energy to sustain HST in normal science operations mode for 7.5 hours, or five orbits. The solar arrays are designed for replacement by visiting astronauts aboard a space shuttle.
In order to run all the many subsystems onboard HST, several computers and microprocessors reside in the body of HST, as well as in each science instrument. Two main computers, which girdle HST's “waist,” direct all operations. One communicates with the instruments, receives their data and telemetry, sends the data to interface units for transmission to the ground, and sends commands and timing information to the instruments. The other main computer handles the gyroscopes, the pointing control sub-system, and other HST-wide functions. Each instrument itself also houses small computers and microprocessors that direct their activities. These computers direct the rotation of the filter wheels, open and close exposure shutters, maintain the temperature of the instruments, collect data, and communicate with the main computers.
In space, HST is subject to the harsh environment of zero gravity and temperature extremes—more than 100 degrees Fahrenheit difference in temperature during each trip around earth. To accommodate this operating environment, HST has a “skin,” or blanket, of multilayered insulation, which protects the telescope during temperature shifts. Beneath this insulation is a lightweight aluminum shell, which provides an external structure for the spacecraft and houses the OTA and science instruments. The OTA is held together by a cylindrical truss made of graphite epoxy, the same material used to make many golf clubs, tennis racquets and bicycles. Graphite is a stiff, strong and lightweight material that resists expanding and contracting in extreme temperatures.
The following table summarizes some of the relevant facts about HST:
TABLE 1Weight24,500 Lbs.Length43.5 Ft.Diameter14 Ft. (Aft Shroud)Optical SystemRitchey-Chretien Design Cassegrain TelescopePrimary Mirror94.5 Inch Dia.Pointing Accuracy0.007 Arcsec for 24 HoursMagnitude Range5 Meters to 30 Meters (Visual Magnitude)Wavelength Range1,100 Angstroms to 24,000 AngstromsAngular Resolution0.1 Arcsec at 6328 AngstromsOrbit320 Nautical Miles Inclined at 28.5 DegreesOrbit Time97 Minutes per Orbit
As indicated by the foregoing narrative, HST was designed to be serviced and upgraded periodically throughout its lifetime. Specifically, as illustrated in FIGS. 5 and 6, the space shuttle program had planned missions dedicated to servicing and upgrading HST scheduled until 2010, at which time HST would be retired and de-orbited in favor of the new James Webb Space Telescope (JWST). However, in January of 2004, NASA Administrator Sean O'Keefe announced that he was canceling shuttle Service Mission 4 (SM4) because of safety issues identified by the Columbia Accident Investigation Board (CAIB) Report. The CAIB Report was issued as a result of the Space Shuttle Columbia disaster of Feb. 1, 2003, when all seven astronauts aboard Columbia were tragically killed on reentry into the atmosphere.
Administrator O'Keefe's announcement presented the scientific community with at least two problems. First, without SM4, regularly scheduled upgrades to HST's scientific instruments could not be made, thereby confining scientific use of HST to out-dated technology. Second, and of graver concern, without SM4, wearable parts on HST could not be replaced. More specifically, gyroscopes necessary for HST's Pointing Control System (PCS) are degrading and will probably cease to operate within the next three to five years, as indicated by the chart of FIG. 7. The current PCS requires sensing from three gyros, and already only four of the six gyros aboard HST are operational. Best estimates show that a less than 50 percent probability exists that three gyros will be operational by the late 2005 or early 2006. Although scientists are developing a two-gyro pointing system, that solution may add only 12 to 18 months of additional life.
Moreover, batteries that power HST's computers, instruments and virtually all vital systems are at risk. Based on recent tests, each of HST's six batteries is losing its charging capacity at a rate of 5.9 amp hours per year. This is far higher than previous tested loss rates of about two amp hours per year, and points out a tendency for capacity loss rate to accelerate nonlinearly over time. Without intervention, as scheduled by SM4, it is projected that by the end of 2005 science operations will likely require block scheduling and a lowering of the safemode trigger. By 2006, it is probable that the state-of-charge at the end of orbit night will be near the hardware sunpoint safemode trigger, which is the lowest level of safemode that protects the vehicle.
Accordingly, there is a need for a method and attendant apparatus for autonomously servicing HST and other free-flying satellites during flight using robotics.